Recently telecommunication networks have been implemented using packet switched connections, in contrast to circuit switched connections, in order to more effectively utilize communication links and to improve throughput efficiency. One type of packet switched connection utilizes the so-called asynchronous transfer mode (ATM). This mode involves asynchronous time division multiplexing and high-speed packet switching. In essence, ATM transports information in the form of specialized packets referred to as "cells". Each ATM cell includes a header followed by accompanying data. The header contains a label which is used for multiplexing and routing.
As an example of a communication service utilizing an ATM network, a video channel is considered. Broadly, such a channel essentially includes: a transmitter; a receiver; and an ATM network link. The ATM network link is provided by the ATM network which is composed of switching elements, resource managers, and physical signal propagation media such as fiber optic cables, interconnecting the transmitter with the receiver. The transmitter is composed of a video camera which is connected to a coder that encodes and generates bits for each video frame passed to it by the camera. The frame generation rate is, for example, 25 Hz or the U.S. standard of 30 Hz. The coder generates a variable number of bits for each frame. These generated bits are buffered with a buffer device in the transmitter. The buffer converts the bit stream to ATM cells and then the cells propagate onto the medium through an ATM packet switched network coupling the buffer to the medium. The propagation medium transports the cells to the receiver where the original information bits are detected via a receiver including a decoder and a display monitor.
Much research attention has been focussed on methods for providing such video transmission using ATM networks, such as described by numerous articles in the: Proceedings of the Seventh Specialist Seminar, International Teletraffic Conference, October 1990; Proceedings of the Third International Workshop on Packet Video, Morristown, N.J., March 1990; and IEEE Journal on Selected Areas of Communication, June 1989. A central issue which has been the focus of numerous investigations is whether variable bit rate (VBR) schemes are better than constant bit rate schemes (CBR). In both CBR and VBR the number of bits varies from frame to frame. In CBR, bits are transmitted at a constant rate per frame, so that different frames take different amounts of time to be transmitted. In VBR, the approach is to transmit as many bits as possible in each frame interval, so that a highly variable rate results. Occasionally, the constant bit rate forces a decrease in picture quality because the bandwidth is too low to handle quickly changing scenes, and fairly often the constant bit rate forces the sending of superfluous bits because slowly changing pictures can be represented with much fewer bits. In VBR, the approach is to transmit only the bits really needed, and (in principle) to transmit all bits from a frame within that frame interval, or very soon afterwards. As a result, VBR has lower average need for bandwidth, but a much higher variability in the traffic.
For efficient use of network resources it may be necessary to multiplex several video sources onto the same transmission link. Then, an important parameter determining the usefulness of VBR schemes is the statistical multiplexing gain defined as the ratio of the number of multiplexed VBR sources to the number of multiplexed CBR sources while maintaining the same subjective picture quality (a definition proposed by H. Heeke in the paper "Statistical Multiplexing Gain for Variable Bit Rate Video Codecs in ATM networks," Proc. 4th International Packet Video Workshop, Tokyo, Japan, March 1991). The picture quality degrades with increases in cell losses. Hence, if the number of multiplexed sources can be increased without increasing cell losses (or cell losses reduced for a given number of sources) the statistical multiplexing gain should increase.
The traffic offered to the network can be made more regular by smoothing. Essentially, in smoothing such traffic, frames consisting of more bits (or cells) take longer to be transmitted than frames consisting of fewer bits (or cells). A main disadvantage of such a smoothing scheme is added delay of the information at the smoothing point. For interactive video such as video conferencing this may be a serious drawback.
The prior art has not disclosed or suggested what could be considered a compromise in traffic smoothing situations, namely, an approach to smoothing whereby, for delay sensitive traffic, an upper bound is imposed on the added delay, and the traffic is smoothed as much as possible subject to that constraint. This approach demands (at least partial) fore-knowledge of traffic arriving at the smoothing point: if a lull in the traffic is imminent then traffic already waiting at the smoothing point must be spread out until the largest allowed delay, while if a peak is imminent then traffic already available must be sent at a higher rate to prevent the need for an even higher rate later.